Inductively rechargeable external energy source, charger, system and method for a transcutaneous inductive charger for an implantable medical device

ABSTRACT

Techniques for transcutaneous transferral of energy to an implantable medical device are disclosed. An embodiment includes a system comprising an implantable medical device having a secondary coil. An external device is provided to transcutaneously transfer energy to the secondary coil. The external device comprises a housing having a side adapted to be positioned in proximity to the secondary coil when the external device is transferring energy to the secondary coil. A temperature sensor is coupled to the side to determine a temperature indicative of heat to which the patient is being exposed during the transfer of energy. A control circuit is adapted to control the transfer of energy to the secondary coil based on the temperature. For instance, the control circuit may limit transfer of energy by controlling times at which transfer of energy occurs or controlling an amplitude of a signal within the external device.

RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 11/687,061 filed Mar. 16, 2007, which is acontinuation of, and claims priority to U.S. patent application Ser. No.10/836,318 filed Apr. 30, 2004, which claims priority toprovisionally-filed U.S. Patent Application 60/508,204 filed Oct. 2,2003, all of which are incorporated herein by reference in theirentireties.

This application is related to, and has some subject matter in commonwith, U.S. patent application Ser. No. 10/836,101 filed Apr. 30, 2004(now U.S. Pat. No. 7,225,032), which claims priority toprovisionally-filed U.S. Patent Application 60/508,097 filed on Oct. 2,2003.

TECHNICAL FIELD

This invention relates to implantable medical devices and, inparticular, to energy transfer devices, systems and methods forimplantable medical devices.

BACKGROUND

Implantable medical devices for producing a therapeutic result in apatient are well known. Examples of such implantable medical devicesinclude implantable drug infusion pumps, implantable neurostimulators,implantable cardioverters, implantable cardiac pacemakers, implantabledefibrillators and cochlear implants. Of course, it is recognized thatother implantable medical devices are envisioned which utilize energydelivered or transferred from an external device.

A common element in all of these implantable medical devices is the needfor electrical power in the implanted medical device. The implantedmedical device requires electrical power to perform its therapeuticfunction whether it be driving an electrical infusion pump, providing anelectrical neurostimulation pulse or providing an electrical cardiacstimulation pulse. This electrical power is derived from a power source.

Typically, a power source for an implantable medical device can take oneof two forms. The first form utilizes an external power source thattranscutaneously delivers energy via wires or radio frequency energy.Having electrical wires which perforate the skin is disadvantageous due,in part, to the risk of infection. Further, continuously couplingpatients to an external power for therapy is, at least, a largeinconvenience. The second form utilizes single cell batteries as thesource of energy of the implantable medical device. This can beeffective for low power applications, such as pacing devices. However,such single cell batteries usually do not supply the lasting powerrequired to perform new therapies in newer implantable medical devices.In some cases, such as an implantable artificial heart, a single cellbattery might last the patient only a few hours. In other, less extremecases, a single cell unit might expel all or nearly all of its energy inless than a year. This is not desirable due to the need to explant andre-implant the implantable medical device or a portion of the device.One solution is for electrical power to be transcutaneously transferredthrough the use of inductive coupling. Such electrical power or energycan optionally be stored in a rechargeable battery. In this form, aninternal power source, such as a battery, can be used for directelectrical power to the implanted medical device. When the battery hasexpended, or nearly expended, its capacity, the battery can be rechargedtranscutaneously, via inductive coupling from an external power sourcetemporarily positioned on the surface of the skin.

Several systems and methods have been used for transcutaneouslyinductively recharging a rechargeable used in an implantable medicaldevice.

U.S. Pat. No. 5,411,537, Munshi et al, Rechargeable Biomedical BatteryPowered Devices With Recharging and Control System Therefor,(Intermedics, Inc.) discloses a hermetically-sealed automaticimplantable cardioverter-defibrillator (AICD) or any otherbioimplantable device which may be operated on a single rechargeablecell, or a dual power source system, the rechargeable complement beingrecharged by magnetic induction. Included in the implantable devices arelithium rechargeable chemistries designed to sense the state-of-chargeor discharge of the battery; a battery charge controller specificallydesigned to recharge a lithium battery rapidly to less than 100% fullcharge, and preferably 90%, more preferably 80%, of full rated chargecapacity; and charging means for multi-step charging. The batteries arebased on lithium chemistries specially designed to yield higher currentsthan conventional primary lithium chemistries and to permit long-termperformance despite sub-capacity recharging.

U.S. Pat. No. 5,690,693, Wang et al, Transcutaneous Energy TransmissionCircuit For Implantable Medical Device, (Sulzer Intermedics Inc.)discloses a transcutaneous energy transmission device for chargingrechargeable batteries in an implanted medical device. A current with asinusoidal waveform is applied to a resonant circuit comprising aprimary coil and a capacitor. Current is induced in a secondary coilattached to the implanted medical device. Two solid-state switches areused to generate the sinusoidal waveform by alternately switching on andoff input voltage to the resonant circuit. The sinusoidal waveformreduces eddy current effects in the implanted device which detrimentallyincreases the temperature of the implanted device. The batteries arecharged using a charging protocol that reduces charging current as thecharge level in the battery increases. The controller is constructed asa pulse with modulation device with a variable duty cycle to control thecurrent level applied to the primary coil. An alignment indicator isalso provided to insure proper and alignment between the energytransmission device and the implanted medical device.

U.S. Pat. No. 5,733,313, Barreras, Sr., FR Coupled Implantable MedicalDevice With Rechargeable Back-Up Power Source, (Exonix Corporation)discloses an implantable, electrically operated medical device systemhaving an implanted radio frequency (RF) receiving unit (receiver)incorporating a back-up rechargeable power supply and an implanted,electrically operated device, and an external RF transmitting unit(transmitter). RF energy is transmitted by the transmitter and iscoupled into the receiver which is used to power the implanted medicaldevice and/or recharge the back-up power supply. The back-up powersupply within the receiver has enough capacity to be able to, by itself,power the implanted device coupled to the receiver for at least 24 hoursduring continual delivery of medical therapy. The receiver is surgicallyimplanted within the patient and the transmitter is worn externally bythe patient. The transmitter can be powered by either a rechargeable ornon-rechargeable battery. In a first mode of operation, the transmitterwill supply power, via RF coupled energy, to operate the receiver andsimultaneously recharge the back-up power supply. In a second mode ofoperation, the receiver can, automatically or upon external command fromthe transmitter, acquire its supply of power exclusively from theback-up power supply. Yet, in a third mode of operation, the receivercan, automatically or upon command from the transmitter, alternativelyacquire it supply of power from either, FR energy coupled into thereceiver or the internal back-up power supply.

U.S. Pat. No. 6,308,101, Faltys et al, Fully Implantable CochlearImplant System, (Advanced Bionics Corporation) discloses a fullyimplantable cochlear implant system and method including an implantablecochlear stimulator unit that is connected to an implantable speechprocessor unit. Both the speech processor unit and the cochlearstimulator unit are in separate, hermetically-sealed, cases. Thecochlear stimulator unit has a coil permanently connected theretothrough which magnetic or inductive coupling may occur with a similarcoil located externally during recharging, programming, orexternally-controlled modes of operation. The cochlear stimulator unitfurther has a cochlear electrode array permanently connected thereto viaa first multi-conductor cable. The cochlear stimulator unit also has asecond multi-conductor cable attached thereto, which second cablecontains no more than five conductors. The second cable is detachablyconnected to the speech processor unit. The speech processor unitincludes an implantable subcutaneous microphone as an integral partthereof, and further includes speech processing circuitry and areplenishable power source, e.g., a rechargeable battery.

U.S. Pat. No. 6,324,430, Zarinetchi et al, Magnetic Shield For PrimaryCoil of Transcutaneous Energy Transfer Device, (Abiomed, Inc.) disclosesa transcutaneous energy transfer device which has a magnetic shieldcovering the primary winding of the device to reduce sensitivity of thedevice to conducting objects in the vicinity of the coils and toincrease the percentage of magnetic field generated by the primary coilwhich reaches the secondary coil. The shield is preferably larger thanthe primary coil in all dimensions and is either formed of a highpermeability flexible material, for example a low loss magnetic materialand a flexible polymer matrix, with perforations formed in the materialsufficient to permit ventilation of the patient's skin situated underthe shield, or the shield may be formed of segments of a very highpermeability material connected by a flexible, porous mesh material.

U.S. Pat. No. 6,516,227, Meadows et al, Rechargeable Spinal CordStimulator System, (Advanced Bionics Corporation) discloses a spinalcord stimulation system providing multiple stimulation channels, eachcapable of producing up to 10 milliamperes of current into a one kilohmload. The system further includes a replenishable power supply, e.g., arechargeable battery that requires only an occasional recharge, andoffers a life of at least 10 years at typical settings. Thereplenishable power source may be replenished using non-invasive means.The system monitors the state of charge of the internal power source andcontrols the charging process by monitoring the amount of energy used bythe system, and hence the state of the charge of power source. Asuitable bidirectional telemetry link allows the system to inform thepatient or clinician regarding the status of the system, including thestate of the charge, and makes requests to initiate an external chargeprocess.

U.S. Pat. No. 6,505,077, Kast et al, Implantable Medical Device WithExternal Recharging Coil Electrical Connection, (Medtronic, Inc.)discloses a rechargeable implantable medical device with an improvedexternal recharging coil electrical connection resistant to corrosion.The electrical connection couples the external recharging coil to arecharge feedthrough. The rechargeable implantable medical device can bea medical device such as a neuro stimulator, drug delivery pump,pacemaker, defibrillator, diagnostic recorder, cochlear implant, and thelike. The implantable medical device has a housing, electronics carriedin the housing configured to perform a medical therapy, a rechargeablepower source, and a recharging coil.

European Patent Application 1,048,324, Schallhorn, MedicalLi+Rechargeable Powered Implantable Stimulator, (Medtronic, Inc.)discloses an implantable stimulator having a rechargeable lithium ionpower source and delivers electrical stimulation pulses, in a controlledmanner, to a targeted site within a patient. The lithium ion powersource can supply sufficient power to the implantable stimulator on anexclusive basis over at least about 4 days. The power source includes ahigh value, small size lithium ion storage unit having a power rating ofat least 50 milliamp hours. The implantable stimulator also has aninductor adapted to gather EMF power transmissions. The implantablestimulator can be replenished with electrical power by an electricalpower replenisher, external to the implantable stimulator, to replenishthe lithium ion power source up to its maximum rated voltage bygenerating the EMF power transmission near the inductor.

PCT Patent Application No. WO 01/83029 A1, Torgerson et al, BatteryRecharge Management For an Implantable Medical Device, (Medtronic, Inc.)discloses an implantable medical device having an implantable powersource such as a rechargeable lithium ion battery. The implantablemedical device includes a recharge module that regulates the rechargingprocess of the implantable power source using closed-loop feedbackcontrol. The recharging module includes a recharge regulator, a rechargemeasurement device monitoring at least one recharge parameter, and arecharge regulation control unit for regulating the recharge energydelivered to the power source in response to the recharge measurementdevice. The recharge module adjusts the energy provided to the powersource to ensure that the power source is being recharged under safelevels.

PCT Patent Application No. WO 01/97908 A2, Jimenez et al, An ImplantableMedical Device With Recharging Coil Magnetic Shield, (Medtronic, Inc.)discloses a rechargeable implantable medical device with a magneticshield placed on the distal side of a secondary recharging coil toimprove recharging efficiency. The rechargeable implantable medicaldevice can be wide variety of medical devices such as neurostimulators,drug delivery pumps, pacemakers, defibrillators, diagnostic recorders,and cochlear implants the implantable medical device has a secondaryrecharging coil carried over a magnetic shield and coupled toelectronics and a rechargeable power source carried inside the housingelectronics are configured to perform a medical therapy. Additionally amethod of for enhancing electromagnetic coupling during recharging of animplantable medical device is disclosed, and a method for reducingtemperature rise during recharging of an implantable medical device isdisclosed.

Transcutaneous energy transfer through the use of inductive couplinginvolves the placement of two coils positioned in close proximity toeach other on opposite sides of the cutaneous boundary. The internalcoil, or secondary coil, is part of or otherwise electrically associatedwith the implanted medical device. The external coil, or primary coil,is associated with the external power source or external charger, orrecharger. The primary coil is driven with an alternating current. Acurrent is induced in the secondary coil through inductive coupling.This current can then be used to power the implanted medical device orto charge, or recharge, an internal power source, or a combination ofthe two.

For implanted medical devices, the efficiency at which energy istranscutaneously transferred is crucial. First, the inductive coupling,while inductively inducing a current in the secondary coil, also has atendency to heat surrounding components and tissue. The amount ofheating of surrounding tissue, if excessive, can be deleterious. Sinceheating of surrounding tissue is limited, so also is the amount ofenergy transfer which can be accomplished per unit time. The higher theefficiency of energy transfer, the more energy can be transferred whileat the same time limiting the heating of surrounding components andtissue. Second, it is desirable to limit the amount of time required toachieve a desired charge, or recharge, of an internal power source.While charging, or recharging, is occurring the patient necessarily hasan external encumbrance attached to their body. This attachment mayimpair the patient's mobility and limit the patient's comfort. Thehigher the efficiency of the energy transfer system, the faster thedesired charging, or recharging, can be accomplished limiting theinconvenience to the patient.

Third, amount of charging, or recharging, can be limited by the amountof time required for charging, or recharging. Since the patient istypically inconvenienced during such charging, or recharging, there is apractical limit on the amount of time during which charging, orrecharging, should occur. Hence, the size of the internal power sourcecan be effectively limited by the amount of energy which can betransferred within the amount of charging time. The higher theefficiency of the energy transfer system, the greater amount of energywhich can be transferred and, hence, the greater the practical size ofthe internal power source. This allows the use of implantable medicaldevices having higher power use requirements and providing greatertherapeutic advantage to the patient and/or extends the time betweencharging effectively increasing patient comfort.

SUMMARY

An improved mechanism for transcutaneously transferring energy from anexternal device (e.g., an external power source) to an implantablemedical device is disclosed. In one embodiment, a system comprising animplantable medical device having a secondary coil and a rechargeablepower source is provided. The system also comprises an external devicehaving a primary coil adapted to inductively couple to the secondarycoil of the implantable medical device to transcutaneously recharge therechargeable power source of the implantable medical device. Theexternal device has a surface that assumes a position in proximity tothe secondary coil when the primary coil is inductively coupled to thesecondary coil. In one scenario, the surface is adapted to be placedagainst the patient's skin or clothing. A temperature sensor may becoupled to the surface of the external device. In one embodiment, thetemperature sensor is thermally-coupled to this surface. The temperaturesensor may reside within a housing of the external device and besituated in proximity to the surface. In some embodiments, at least aportion of the surface may be thermally conductive.

The system may further comprise control circuitry that isoperationally-coupled to the temperature sensor to control recharging ofthe rechargeable power source based on an output from the temperaturesensor. In one embodiment, the control circuitry may comprise aprogrammed-controlled processor. The control circuitry may be adapted tolimit energy transfer between the external device and the secondarycoil. For instance, the control circuitry may be adapted to control aduty cycle of energy transfer between the external device and thesecondary coil for recharging of the rechargeable power source based onthe output from the temperature sensor.

The external device may be adapted to limit at least one of atemperature of the surface and a temperature of a surface of a patientin which the implantable medical device is implanted to no higher than arespective predetermined temperature. The external device of anillustrated embodiment may further comprise an adjustable assemblyadapted to adjust efficiency of energy transfer between the primary coiland the secondary coil.

In some cases, the external device may be adapted to exchange data withthe implantable medical device. For instance, this may involve engagingin telemetry communication with the implantable medical device. Theexternal device may further comprise a circuit adapted to monitorrecharging of the rechargeable power source and to provide status of therecharging to a user.

Another embodiment relates to a system comprising an implantable medicaldevice adapted to be implanted within a patient. The implantable medicaldevice comprises a rechargeable power source and a secondary coilcoupled to the rechargeable power source. The system may also include anexternal device comprising a housing having a surface adapted to assumea position in proximity to a surface of the patient during recharging ofthe rechargeable power source. This surface of the housing may beadapted to contact the patient during recharging of the rechargeablepower source.

The external device may further include a primary coil adapted to betranscutaneously coupled to the secondary coil during the rechargingoperation. A temperature sensor may be provided to sense a temperatureof the surface of the housing during the recharging. Thermallyconductive material may, in one case, be provided in close proximity tothe temperature sensor. The external device may comprise a circuit tovary a frequency of a recharging signal to substantially optimizeefficiency of energy transfer between the primary coil and the secondarycoil. The external device may further comprise a telemetry coil adaptedto exchange communication signals with the implantable medical device.

The external device may, in one scenario, comprise control circuitry,which may include a processor. The control circuitry may control (and insome cases may limit) the recharging based on the temperature of thesurface of the housing. The control circuitry may be adapted to controlenergy transfer to the implantable medical device during the rechargingby limiting time during which energy is transferred. The controlcircuitry may be adapted to switch energy transfer on and off to providean energy transfer duty cycle. In some embodiments, the controlcircuitry may be adapted to limit the temperature of the surface of thehousing to no more than a predetermined temperature.

Still another aspect relates to a system comprising an implantablemedical device implantable within a patient and comprising a secondarycoil. The system may also include an external device to transcutaneouslytransfer energy to the secondary coil. The external device may comprisea housing having a side adapted to be positioned in proximity to thesecondary coil when the external device is transferring energy to thesecondary coil. A temperature sensor coupled to the side determines atemperature indicative of heat to which the patient is being exposedduring the transfer of energy. This temperature sensor may be carriedwithin the housing of the external device, and may be directly orindirectly coupled to the side. A control circuit may be adapted tolimit the transfer of energy to the secondary coil based on thetemperature. The control circuit may be adapted to control transfer ofenergy so that the temperature does not exceed a predetermined maximumtemperature. The control circuit may further comprise a processorexecuting programmed instructions, and the predetermined maximumtemperature may be selectable by the processor.

Control over the energy transfer may be accomplished by limiting timeduring which the energy is transferred, controlling frequency at whichenergy is transferred, and/or as by controlling an amplitude of a signalwithin the primary coil (e.g., amplitude of a current or a voltagewithin the primary coil.).

In one embodiment, the temperature sensor is thermally-coupled to asurface of the side of the housing, the surface being in contact withthe patient. In some cases, at least a portion of the side of thehousing may carry a thermally-conductive material.

The external device may further include a circuit to vary a frequency ofa signal used to transfer energy to the secondary coil to substantiallyoptimize efficiency of the transfer of energy.

In some embodiments, the implantable medical device further comprisescircuitry to deliver therapy to the patient and magnetic materialpositioned between the secondary coil and at least a portion of thecircuitry to deliver therapy.

Another embodiment involves a computer-readable medium comprisinginstructions that, upon execution, cause a processor to controltranscutaneous transfer of energy from an external device to a secondarycoil of an implantable medical device while a side of the externaldevice assumes a position in proximity to the secondary coil. Theinstructions may further cause the processor to receive output from atemperature sensor that is coupled to the side of the external deviceand to control the transcutaneous transfer of energy based, at least inpart, on the received output. The received output may reflect a thermalcoupling of the temperature sensor to the side of the external device

According to this embodiment, the instructions may, in one case, furthercause the processor to limit energy transfer between the external deviceand the secondary coil. For instance, the processor may control a dutycycle of energy transfer between the external device and the secondarycoil. The processor may alternatively or additionally limit at least oneof a temperature of the side of the external device and a temperature ofa surface of a patient in which the implantable medical device isimplanted to no higher than a respective predetermined temperature. Theinstructions may cause the processor to adjust efficiency of energytransfer to the secondary coil, as by causing the processor to adjust afrequency of the energy transfer to the secondary coil. The instructionsmay, in some embodiments, result in the processor switching energytransfer on and off to control the energy transfer to the secondarycoil. The instructions may further cause the processor to provide statusof the energy transfer between the external device and the secondarycoil to a user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable medical device implanted in a patient;

FIG. 2 is a block diagram of an implantable medical device;

FIG. 3 is a detailed block diagram of an implantable medical deviceimplanted sub-cutaneously and an associated external charging device inaccordance with an embodiment of the present invention;

FIG. 4 is a perspective view of an internal antenna associated with animplantable medical device;

FIG. 5 is a side view of the internal antenna of FIG. 4;

FIG. 6 is an exploded perspective view an external antenna andassociated bracket in accordance with an embodiment of the presentinvention;

FIG. 7 is a top view of an external antenna in accordance with anembodiment of the present invention;

FIG. 8 is a perspective view of an external antenna and bracketcombination in accordance with an embodiment of the present invention;

FIG. 9 is a cross-sectional side view of an implantable medical deviceimplanted sub-cutaneously and an associated bracket for use with anexternal antenna;

FIG. 10 is a cut-away top view of view a primary coil and associatedmagnetic core in accordance with an embodiment of the present invention;

FIG. 11 is a cross-sectional view of the primary coil and associatedmagnetic core of FIG. 10 taken through section line B-B;

FIG. 12 is an exploded view a portion of an external antenna constructedin accordance with an embodiment of the present invention showing themagnetic core and a core cup assembly;

FIG. 13 is block diagram of an external charging unit and an associatedinductively coupled cradle for recharging the external charging unit;

FIG. 14 is a detailed block diagram of the external charging unit ofFIG. 13;

FIG. 15 is a flow chart illustrating a charging process in accordancewith an embodiment of the present invention; and

FIG. 16 is a schematic diagram of a dual range temperature sensor.

DETAILED DESCRIPTION

FIG. 1 shows implantable medical device 16, for example, a drug pump,implanted in patient 18. The implantable medical device 16 is typicallyimplanted by a surgeon in a sterile surgical procedure performed underlocal, regional, or general anesthesia. Before implanting the medicaldevice 16, a catheter 22 is typically implanted with the distal endposition at a desired therapeutic delivery site 23 and the proximal endtunneled under the skin to the location where the medical device 16 isto be implanted. Implantable medical device 16 is generally implantedsubcutaneously at depths, depending upon application and device 16, offrom 1 centimeter (0.4 inches) to 2.5 centimeters (1 inch) where thereis sufficient tissue to support the implanted system. Once medicaldevice 16 is implanted into the patient 18, the incision can be suturedclosed and medical device 16 can begin operation.

Implantable medical device 16 operates to infuse a therapeutic substanceinto patient 18. Implantable medical device 16 can be used for a widevariety of therapies such as pain, spasticity, cancer, and many othermedical conditions.

The therapeutic substance contained in implantable medical device 16 isa substance intended to have a therapeutic effect such as pharmaceuticalcompositions, genetic materials, biologics, and other substances.Pharmaceutical compositions are chemical formulations intended to have atherapeutic effect such as intrathecal antispasmodics, pain medications,chemotherapeutic agents, and the like. Pharmaceutical compositions areoften configured to function in an implanted environment withcharacteristics such as stability at body temperature to retaintherapeutic qualities, concentration to reduce the frequency ofreplenishment, and the like. Genetic materials are substances intendedto have a direct or indirect genetic therapeutic effect such as geneticvectors, genetic regulator elements, genetic structural elements, DNA,and the like. Biologics are substances that are living matter or derivedfrom living matter intended to have a therapeutic effect such as stemcells, platelets, hormones, biologically produced chemicals, and thelike. Other substances may or may not be intended to have a therapeuticeffect and are not easily classified such as saline solution,fluoroscopy agents, disease diagnostic agents and the like. Unlessotherwise noted in the following paragraphs, a drug is synonymous withany therapeutic, diagnostic, or other substance that is delivered by theimplantable infusion device.

Implantable medical device 16 can be any of a number of medical devicessuch as an implantable therapeutic substance delivery device,implantable drug pump, cardiac pacemaker, cardioverter or defibrillator,as examples.

In FIG. 2, implantable medical device 16 has a rechargeable power source24, such as a Lithium ion battery, powering electronics 26 and therapymodule 28 in a conventional manner. Therapy module 28 is coupled topatient 18 through one or more therapy connections 30, alsoconventionally. Rechargeable power source 24, electronics 26 and therapymodule 28 are contained in hermetically sealed housing 32. Secondarycharging coil 34 is attached to the exterior of housing 32. Secondarycharging coil 34 is operatively coupled through electronics 26 torechargeable power source 24. In an alternative embodiment, secondarycharging coil 34 could be contained in housing 32 or could be containedin a separate housing umbilically connected to electronics 26.Electronics 26 help provide control of the charging rate of rechargeablepower source 24 in a conventional manner. Magnetic shield 36 ispositioned between secondary charging coil 34 and housing 32 in order toprotect rechargeable power source 24, electronics 26 and therapy module28 from electromagnetic energy when secondary charging coil 34 isutilized to charge rechargeable power source 24.

Rechargeable power source 24 can be any of a variety power sourcesincluding a chemically based battery or a capacitor. In a preferredembodiment, rechargeable power source is a well known lithium ionbattery.

FIG. 3 illustrates an alternative embodiment of implantable medicaldevice 16 situated under cutaneous boundary 38. Implantable medicaldevice 16 is similar to the embodiment illustrated in FIG. 2. However,charging regulation module 42 is shown separate from electronics 26controlling therapy module 28. Again, charging regulation and therapycontrol is conventional. Implantable medical device 16 also has internaltelemetry coil 44 configured in conventional manner to communicatethrough external telemetry coil 46 to an external programming device(not shown), charging unit 50 or other device in a conventional mannerin order to both program and control implantable medical device and toexternally obtain information from implantable medical device 16 onceimplantable medical device has been implanted. Internal telemetry coil44, rectangular in shape with dimensions of 1.85 inches (4.7centimeters) by 1.89 inches (4.8 centimeters) constructed from 150 turnsof 43 AWG wire, is sized to be larger than the diameter of secondarycharging coil 34. Secondary coil 34 is constructed with 182 turns of 30AWG wire with an inside diameter of 0.72 inches (1.83 centimeters) andan outside diameter of 1.43 inches (3.63 centimeters) with a height of0.075 inches (0.19 centimeters). Magnetic shield 36 is positionedbetween secondary charging coil 34 and housing 32 and sized to cover thefootprint of secondary charging coil 34.

Internal telemetry coil 44, having a larger diameter than secondary coil34, is not completely covered by magnetic shield 36 allowing implantablemedical device 16 to communicate with the external programming devicewith internal telemetry coil 44 in spite of the presence of magneticshield 36.

Rechargeable power source 24 can be charged while implantable medicaldevice 16 is in place in a patient through the use of external chargingdevice 48. In a preferred embodiment, external charging device 48consists of charging unit 50 and external antenna 52. Charging unit 50contains the electronics necessary to drive primary coil 54 with anoscillating current in order to induce current in secondary coil 34 whenprimary coil 54 is placed in the proximity of secondary coil 34.Charging unit 50 is operatively coupled to primary coil by cable 56. Inan alternative embodiment, charging unit 50 and antenna 52 may becombined into a single unit. Antenna 52 may also optionally containexternal telemetry coil 46 which may be operatively coupled to chargingunit 50 if it is desired to communicate to or from implantable medicaldevice 16 with external charging device 48. Alternatively, antenna 52may optionally contain external telemetry coil 46 which can beoperatively coupled to an external programming device, eitherindividually or together with external charging unit 48.

As will be explained in more detail below, repositionable magnetic core58 can help to focus electromagnetic energy from primary coil 46 to moreclosely be aligned with secondary coil 34. Also as will be explained inmore detail below, energy absorptive material 60 can help to absorb heatbuild-up in external antenna 52 which will also help allow for a lowertemperature in implantable medical device 16 and/or help lower rechargetimes. Also as will be explained in more detail below, thermallyconductive material 62 is positioned covering at least a portion of thesurface of external antenna 52 which contacts cutaneous boundary 38 ofpatient 18.

In a preferred embodiment of internal antenna 68 as shown in FIG. 4 andFIG. 5, secondary coil 34 and magnetic shield 36 are separate from butadjacent to housing 32 encompassing the remainder of implantable medicaldevice 16. Internal antenna 68 is contained in a separate housing 74which is attachable to housing 32 so that implantable medical device 16can be implanted by a medical professional as essentially one unit.Secondary coil 34 is electrically attached to charging regulation module42 through leads 82.

In order to achieve efficient inductive coupling between primary coil 54of external antenna 52 and secondary coil 34, it is desirable to placeprimary coil 54 of external antenna 52 as close to secondary coil 34 aspossible. Typically, external antenna 52 is placed directly on cutaneousboundary 38 and, since the location of implantable medical device 16 isfixed, the distance across cutaneous boundary 38 between primary coil 54and secondary coil 34 is minimized as long as external antenna 52 iskept adjacent cutaneous boundary 38.

In a preferred embodiment, external antenna 52 is attachable to patient18 with bracket 84 when charging rechargeable power source 24. FIG. 6 isan exploded illustration of a preferred embodiment of external antenna52 attachable to bracket 84. Primary coil 54 is contained in bobbinassembly 86 which sits in bottom housing 88. Primary coil is connectableto cable 56. The bottom of external antenna 52 is formed from athermally conductive material 90. Rotating core cup assembly 92 is heldin place by top housing 94. Rotating core cup assembly 92 is rotatableis allowed to rotate within external antenna 52. Detents 96 engagedetent spring 98 to position rotatable core cup assembly 92 in one of aplurality of detent positions. External antenna may be secured together,for example, with screws (not shown) holding top housing 94 andthermally conductive material 90 together.

Bracket 84 is adapted to be attached to the body of patient 18 with abelt (not shown) attachable to bracket 84 with belt loops 102. Ears 104are adapted to mate with tabs 106 in top housing 94 and pivotally secureexternal antenna 52 in bracket 84 when charging is to be accomplished.Bracket 84 has an opening 108 allowing thermally conductive material 90of external antenna 52 to contact the skin of patient 18 when externalantenna 52 is pivotally secured in bracket 84.

As bracket 84 is attached to patient 18 with a belt via belt loops 102,the skin surface of patient 18 is typically not completely flat. Forexample, if implantable medical device 16 is implantable in the bodytorso of patient 18, then the belt attached via belt loops 102 willtypically pass around the torso of patient 18. Since the torso ofpatient 18, and especially the torso of patient 18 near the location ofimplantable medical device 16, bracket 84 may not sit completely flat onpatient 18. This may be especially true as patient 18 moves and thetorso flexes during such movement. It is preferred that bracket 84 beconformal and flexible in order to conform to the shape of the body ofpatient 18. However, it is also preferred that bracket 84 be rigidenough so that opening 108 in bracket 84 maintain it shape in order toproperly receive external antenna 52. Bracket 84 is preferablyconstructed of PCABS. To maintain the proper position of bracket 84 withthe skin of patient 18, the surface of bracket 84 closest to patient 18contains material 109 constructed from a high durometer, e.g., 40 ShoreA, or “sticky” material such as a material known under the tradename of“Versaflex” manufactured by GLS Corp. of McHenry, Ill. This will helpexternal antenna to sit more closely to the skin surface of patient 18and remain there during movements of patient 18 throughout the charge orrecharge cycle. In addition, external antenna 52 is allowed to pivot byway of ears 104 on tabs 106. Bracket 84 is configured to allow thermallyconductive material 90 to extend through opening 108 and contact theskin surface of patient 18. Allowed pivoting of external antenna 52 and,hence, thermally conductive material 90, permits thermally conductivesurface to sit more closely to the skin surface of patient 18.

FIG. 7 is a partially cut away top view of external antenna 52 isassembled form and attached to cable 56. Rotatable core cup assembly 92is shown located inside of primary coil 54 and positionable in selectedrotated positions via detents 96 and detent spring 98. In FIG. 7,rotatable core cup assembly is positioned between with detent spring 98between detents 96 illustrating that while multiple detent positions areavailable, rotatable core cup assembly can be positioned between detentpositions and, indeed, at any rotated position.

In FIG. 8, the assembly of external antenna 52 with bracket 84 is shownconnected to cable 56. It is preferred that bracket 84 be affixed topatient 18 through belt loops 102 and then, after bracket 84 has beenaffixed to patient 18, external antenna 52 be attached to bracket 84.Affixing bracket 84 to patient 18 first allows for bracket 84 to be usedto laterally position external antenna close to the position ofimplantable medical device 16.

Typical prior art positioning systems rely on the external antenna forlateral positioning. The external antenna is moved around on the body ofthe patient 18 until the best lateral position is found. When the bestlateral position is found, the external antenna is removed from the bodyand the bottom of the external antenna (the portion of the externalantenna) contacting the patient's body) is made to be resistant tolateral movement. As an example, one way is to remove a protective linerexposing a sticky surface allowing the external antenna to be relativelyfixed in location. However, the very act of lifting the external antennain order to remove the protective liner and replacing the externalantenna on the body of the patient 18 causes crucial positioninginformation to be lost. There is no guarantee, and in fact it is notlikely, that the external antenna will be replaced in the exact sameposition as the position previously found to be best.

In contrast, bracket 84 of the present invention can be used to roughlyfind the optimum position for external antenna 52. This can be donerelatively easily due to opening 108 in bracket 84. Implantable medicaldevice 16, when implanted, usually leaves an area of the body of patient18 which is not quite as flat as it was before implantation. That is,implantable medical device 16 usually leaves an area of the skin ofpatient 18 which bulges somewhat to accommodate the bulk of implantablemedical device 16. It is relatively easy for patient, medicalprofessional or other person, to place bracket 84 in the general area ofimplantable medical device 16 and move bracket 84 around until the bulgecaused by implantable medical device 16 is most closely centered inopening 108. As bracket 84 is moved laterally, opening 108 tends tonaturally center on the bulge created by implantable medical device 16.Once positioned in this manner, bracket 84 can be secured to the body ofpatient 18 with belt (not shown) attached via belt loops 102. Securingand/or tightening, by pulling the belt tight or snapping a buckle, forexample, can be without removing bracket 84 from the body of patient 16.Thus, bracket 84 can be relatively easily positioned over the generallocation of implantable medical device 16 and secured in that positionwithout be removed from the body of patient 18.

FIG. 9 is cross-sectional view of implantable medical device 16implanted in patient 18 approximately one centimeter under cutaneousboundary 38 creating bulging area 110, an area of the body of patient 18in which the skin of patient 18 is caused to bulge slightly due to theimplantation of implantable medical device 16. Bulging area 110 is anaid to locating the position of external antenna 52 relative tosecondary coil 34. Bracket 84 can be positioned roughly in the areawhere implantable medical device 16 is implanted. Opening 108 in bracket84 can aid is establishing the location of implantable medical device.Bracket 84 can be roughly centered over bulging area 110. After externalantenna 52 is coupled to bracket 84, then primary coil 54 can begenerally centered on implantable medical device 16.

However, secondary coil 34 may not be centered with respect toimplantable medical device 16. This can occur due to a variety ofreasons such as the need for operatively coupling secondary coil 34 tocharging regulation module 42. Connections to make this operativecoupling may require physical space on one side of internal antenna 68which may cause secondary coil 34 not to be centered on implantablemedical device 16. It is also possible that the attachment of internalantenna 68 to housing 32 can cause secondary coil 34 not to be centeredon implantable medical device 16. Regardless of the cause, if secondarycoil 34 is not centered on implantable medical device 16, then centeringbracket 84 on bulging area 110 may not optimally position primary coil54 with respect to secondary coil 34. Any offset in the position ofprimary coil 54 and secondary coil 34 may not result in the mostefficient energy transfer from external antenna 52 to implantablemedical device 16.

A magnetic core 58 is positioned within primary coil 54 in order tofocus energy generated by primary coil 54. Magnetic core 58 attracts themagnetic flux lines generated by primary coil 54. The position ofmagnetic core 58 within primary coil 54 the lateral location of thelargest amount of the flux lines generated by primary coil 54. FIGS. 10and 11 show cut-away top and cross-sectional views of magnetic core 58used with primary coil 54. Magnetic core 58 is moveable within primarycoil 54. Lower portion 122 of magnetic core 58 can be rotated to aplurality of positions within primary coil 58 by rotating core cupassembly 92 (see FIG. 12). In a preferred embodiment, the travel path ofmagnetic core 58 can be locked in a plurality of discrete positions. Ina preferred embodiment, magnetic core 58 is locked in four (4) differentpositions by detents 96 and detent spring 98 (see FIG. 6). Magnetic core58 has an upper planar portion 120 and a smaller lower portion 122.

As magnetic core 58 is repositioned within primary coil 54, the focus ofmagnetic flux generated by primary coil 54 is also repositioned. Asnoted above, external antenna 52 is generally aligned with implantedmedical device 16 using palpatory sensation. Moveable magnetic core 58can then be used to provide a “fine” adjustment to the lateralpositioning of external antenna 52 with respect to secondary coil 34.After bracket 84 has been secured to patient 18, external antenna 52 isattached to bracket 84. Magnetic core 58 is then moved until the bestlateral alignment with secondary coil 34.

Magnetic core 58 is shown positioned within external antenna 52 of FIG.12. Core cup assembly 92 holds magnetic core 58 within the assembly ofexternal antenna 52. Lower portion 122 (not visible in FIG. 12) ofmagnetic core 58 fits into recess 124 of core cup assembly 92 whileupper portion 120 of magnetic core 58 rests upon ledge 126 of core cupassembly 92. Preferably, magnetic core 58 is a ferrite core. Still morepreferably, magnetic core 58 is constructed from MN60LL highperformance, low loss ferrite manufactured by Ceramic Magnetics, Inc.,Fairfield, N.J. Magnetic core 58 has an initial permeability of 6,500and a maximum permeability of 10,500 (typical) with a volume resistivityof 500 ohm-centimeters.

One surface, preferably the top, of magnetic core 58 is lined with anadhesive coated foam 127 and contained in core cup assembly 92. Magneticcore 58 has a tendency to be brittle. Containing magnetic core 58 iscore cup assembly assures that even if magnetic core 58 has one or morefractures, magnetic core 58 will still be properly positioned andcontinue to function. Foam 127 also helps to hold magnetic core 58together and minimize gaps between fractured segments of magnetic core58. Further, foam 127 adds mechanical stability to magnetic core 58helping to cushion magnetic core 58 against mechanical impacts, such asfrom dropping external antenna 52 against a hard surface, and helps toprevents audible rattles which may otherwise develop from a fracturedmagnetic core 58.

As shown in FIG. 13, external charging device 48 can be powered eitherdirectly from internal (to charging unit 50) batteries 160 or indirectlyfrom desktop charging device 162. Desktop charging device is connectablevia power cord 164 to a source of AC power, such as a standard readilyavailable wall outlet. Desktop charging device 162 can be configured asa cradle which can receive charging unit 50. Other forms of connectionfrom desktop charging device 162 to a power source, such as by adedicated line cable can also be utilized. Desktop charging device 162can charge and/or recharge batteries 160 in charging unit 50, preferablyby inductive coupling using coil 167 positioned in desktop chargingdevice 162 and coil 168 positioned within charging unit 50. Once chargedand/or recharged, batteries 160 can provide the power through internalcircuitry 166 and cable 56 to external antenna 52. Since charging unit50 is not, in a preferred embodiment, coupled directly to the linevoltage source of AC power, charging unit 50 may be used with externalantenna 52 to transfer power and/or charge implanted medical device 16while desktop charging device 162 is coupled to a line voltage source ofAC power. The inductive coupling using coil 167 and coil 168 break thepossibility of a direct connection between the line voltage source of ACpower and external antenna 52. Batteries 160 also allow charging unit 50and, hence, external charging device 48, to be used in transferringpower and/or charging of implanted medical device 16 while completelydisconnected from either a line voltage source of AC power or desktopcharging device 162. This, at least in part, allows patient 18 to beambulatory while transferring power and/or charging implanted medicaldevice 16.

FIG. 14 is a block diagram of external charging device 48 controlled bymicroprocessor 212. Transmit block 214 consists of an H-bridge circuitpowered from 12 volt power supply 216. Transmit block 214 drives primarycoil 54 in external antenna 52. H-bridge control signals and timing areprovided conventionally by microprocessor 212. H-bridge circuit intransmit block 214 is used to drive both primary coil 54, used for powertransfer and/or charging, and telemetry antenna 218. Drive selection isdone by electronically controllable switch 220. During power transferand/or charging, H-bridge circuit is driven at 9 kiloHertz. Duringtelemetry, H-bridge circuit is driven at 175 kiloHertz.

Receive block 222 is used only during telemetry, enabled by switch 224,to receive uplink signals from implanted medical device 16. Twelve voltpower supply 216 is a switching regulator supplying power to transmitblock 214 during power transfer and/or charging as well as telemetrydownlink. Nominal input voltage to 12 volt power supply 216 is either7.5 volts from lithium ion batteries 226 or 10 volts from desktopcharging device 162 (FIG. 13).

Current measure block 226 measures current to 12 volt power supply 216.Current measured by current measure block 226 is used in the calculationof power in along with the voltage of batteries 160. As noted above,power in is used in the calculation of efficiency of power transferand/or charging efficiency to determine, in part, the best location ofexternal antenna 52 and/or rotating core cup assembly 92.

Rotating core cup assembly 92 is rotated in external antenna 52 forbetter lateral alignment of primary coil 54 and secondary coil 34. Afeedback mechanism is used to determine the best rotation of core cupassembly 92. External charging device 48 can determine whether thecurrent position of rotating core cup assembly 92 is optimally alignedfor energy transfer and/or charging. External charging device 48measures the power out of external charging device 48 divided by thepower into external charging device 48. This calculation is a measure ofthe efficiency of external charging device 48. The power out is gaugedby the power induced in implantable medical device 16 and is determinedby multiplying the voltage of rechargeable power source 24 by thecharging current in implantable medical device 16. These values areobtained by telemetry from implanted medical device 16. The power in isgauged by the power generated by charging unit 50 and is determined bymultiplying the voltage of the internal voltage of charging unit 50,e.g., the voltage of a battery or batteries internal to charging unit50, by the current driving external antenna 52.

The ratio of power out divided by power in can be scaled displayed topatient 18, or a medical professional or other person adjustingrotatable core cup assembly 92 or positioning external antenna 52. Forexample, the available efficiency can be divided into separate rangesand displayed as a bar or as a series of lights. The separate ranges canbe linearly divided or can be logarithmic, for example.

Using efficiency as a measure of effective coupling and, hence, as ameasure of proper location of external antenna 52 and rotatable core cupassembly 92 works not only at high charging or power transfer levels butalso at reduced charging levels, as for example, when charging atreduced levels toward the end or beginning of a charging cycle.

If, after patient 18 or other person has moved rotatable core cupassembly 92 through all of the range of positions on external antenna 52and can not achieve an acceptable efficiency level, patient 18 or otherperson can remove external antenna 52 from bracket 84, realign bracket84 with bulging area 110, reattach external antenna 52 to bracket 84 andrestart the alignment and coupling efficiency process.

FIG. 15 is a flow chart illustrating an exemplary charging process usingexternal antenna 52. The process starts [block 127] and a chargingsession begins [block 128] with a test [block 130]. The charging systemperforms start-up checks [block 132]. If the start-up checks are notperformed successfully, the actions taken in Table 1 are performed.

TABLE 1 Check Screen/Message System Errors: e.g., stuck key System ErrorExternal Charger Battery Status Recharge Complete Battery Low RechargeExternal Charger External Charger Connected to External Recharge inProcess Icon Antenna Antenna Disconnect Connect Antenna

If the start-up checks are successful, telemetry with implantablemedical device 16 is checked [block 134]. If telemetry is successful,the error messages indicated in Table 2 are generated.

TABLE 2 Failure Screen/Message Poor Communication Reposition AntennaExternal Charger Error Code Response Call Manufacturer CommunicationError Communication Error External Charger Fault Call ManufacturerAntenna Disconnect Connect Antenna Antenna Failure Antenna Failure Icon

If telemetry checks are successful, external charging device 48 is ableto monitor [block 136] charging status. Monitoring charging status canincludes providing feedback to an operator to help determine the bestrotational position of core cup assembly 92.

Charge events are checked [block 138]. If no charge events are noted,the actions indicated in Table 3 are executed.

TABLE 3 Event Screen/Message Telemetry Failure (See Messages From Table2) Implantable Medical Device Battery Low Device Battery Low ExternalCharger Battery Low Charger Battery Low External Charger BatteryDepleted Recharge Charger External Charger Recharge Complete ExternalCharger Recharge Complete Implantable Medical Device Will Not RechargeDevice Provide Therapeutic Result Until Recharged: TherapyUnavailable/Sleep Mode Antenna Disconnect Connect Antenna

If a charge event occurs, then the process checks to determine ifcharging is complete [block 140]. Once charging is complete, the processterminates [block 142].

As energy is transferred from primary coil 54 of external antenna 52 tosecondary coil 34 of implantable medical device 16, heat may also begenerated in implantable medical device 16 in surrounding tissue ofpatient 18. Such heat build-up in tissue of patient 18, beyond certainlimits, is undesirable and should be limited as acceptable values.Generally, it is preferable to limit the temperature of external antenna52 to not more than forty-one degrees Centigrade (41° C.) and to limitthe temperature of implanted medical device 16 and the skin of patient18 to thirty-nine degrees Centigrade (39° C.). In order to ensure thatimplantable medical device 16 is less than the upper limit ofthirty-nine degrees Centigrade (39° C.), it is preferred that the actualtemperature of external antenna 52 be less than thirty-nine degreesCentigrade (39° C.). In general, the temperature of external antenna 52should be maintained to be less than or equal to the desired maximumtemperature of implanted medical device 16. While the temperature limitsdiscussed above are preferred under current conditions and regulations,it is recognized and understood that conditions and regulations maychange or be different in different circumstances. Accordingly, theactual temperatures and temperature limits may change. In a preferredembodiment, such temperature limits are under software control incharging unit 50 so that any such temperatures or temperature limits canbe modified to fit the then current circumstances.

Magnetic shield 36 serves to at least partially protect the portion ofimplantable medical device 16 contained within titanium housing 32 fromthe effects of energy transfer from external charging device 48 producedthrough inductive coupling from primary coil 54. Magnetic shield 36 isconstructed of Metglas magnetic alloy 2714A (cobalt-based) manufacturedby Honeywell International, Conway, S.C. Magnetic shield 36 ispositioned between secondary coil 34 and housing 32 of implantablemedical device 16 with secondary coil 34 facing cutaneous boundary 38.Magnetic shield does not interfere with the operation of secondary coil34 because magnetic shield 36 is positioned away from primary coil 54.Also, magnetic shield does not interfere with telemetry betweenimplantable medical device 16 and an external programmer becausemagnetic shield 36 is smaller than internal telemetry coil 44. That is,internal telemetry coil 44 lies outside of magnetic shield 36.

However, the material of magnetic shield 36 substantially limits theelectromagnetic energy induced by primary coil 54 from penetratingbeyond magnetic shield. Electromagnetic waves induced by primary coil 54that reach titanium housing 32 will tend to be absorbed by titaniumhousing 54 and its components and will tend to cause the temperature oftitanium housing 54 to rise. As the temperature of titanium housing 54rises, such temperature increase will be disadvantageously transferredto the surrounding tissue of patient 18. However, any electromagneticwaves which are prevented from reaching titanium housing 32 will notcause such a temperature rise.

Thermally conductive material 62 of external antenna 52 is positioned tocontact the skin of patient 18 when external antenna 52 is placed forenergy transfer, or charging, of implanted medical device 16. Thermallyconductive material 62 tends to spread any heat generated at the skinsurface and spread any such heat over a larger area. Thermallyconductive material 62 tends to make the temperature of the skin surfacemore uniform than would otherwise be the case. Uniformity of temperaturewill tend to limit the maximum temperature of any particular spot on theskin surface. The skin itself is a pretty good conductor of heat andinitially spreading any heat generated over a larger area of the skinwill further assist the skin in dissipating any heat build-up on tosurrounding tissue and further limit the maximum temperature of anyparticular location on the surface of the skin.

Thermally conductive material 62 is molded into the surface of externalantenna 52 which will contact the skin surface of patient 18 whenexternal antenna 52 provides energy transfer to implanted medical device16. Since thermally conductive material 62 should pass electromagneticenergy from primary coil 54, thermally conductive material 62 should beconstructed from a non-magnetic material. It is desirable that thermallyconductive material 62 have a thermal conductivity of approximately 5.62BTU inch/hour feet degrees Fahrenheit (0.81 W/meters degrees Kelvin). Ina preferred embodiment, thermally conductive material is constructedfrom a proprietary composite of approximately forty percent (40%)graphite, seven percent (7%) glass in RTP 199×103410 A polypropylene,manufactured by RTP Company, Winona, Minn. It is also preferable thatthermally conductive material not be electrically conductive in order toreduce eddy currents. In a preferred embodiment, thermally conductivematerial has a volume resistivity of approximately 10³ ohm-centimetersand a surface resistivity of 10⁵ ohms per square.

Energy absorptive material 62 is placed in and/or around primary coil 54of external antenna 52 in order to absorb some of the energy generatedby primary coil 54. In a preferred embodiment, energy absorptivematerial 62 fills in otherwise empty space of rotating core cup assembly92. Heat generated by energy produced by primary coil 54 which is noteffectively inductively coupled to secondary coil 34 will tend to causea temperature rise in other components of external antenna 52 and,possibly, the skin of patient 18. At least a portion of this temperaturerise can be blocked through the use of energy absorptive material 62.Energy absorptive material 62 is chosen to absorb heat build-up insurrounding components and tend to limit further temperature increases.Preferably, energy absorptive material 62 is selected to be materialwhich undergoes a state change at temperatures which are likely to beencountered as the temperature of surrounding components rises duringenergy transfer, e.g., charging, using external antenna 52.

If it is a goal to limit the temperature of the skin of patient 18 tothirty-nine degrees Centigrade (39° C.), it is desirable to use ofenergy absorptive material 62 which has a state change at or near thetemperature limit. In this example, the use of an energy absorptivematerial 62 having a state change in temperature area just belowthirty-nine degrees Centigrade (39° C.), preferably in the range ofthirty-five degrees Centigrade (35° C.) to thirty-eight degreesCentigrade (38° C.), can help limit the rise in the temperature of theskin of patient 18 to no more than the desired limit, in this example,thirty-nine degrees (39° C.).

As the temperature of surrounding components of external antenna 52 riseto a temperature which is just below the temperature at which energyabsorptive material 62 changes state, at least a portion of further heatenergy generated by primary coil 54 and surrounding components ofexternal antenna 52 will go toward providing the energy necessary forenergy absorptive material 62 to change state. As energy absorptivematerial 62 is in the process of changing state, its temperature is notincreasing. Therefore, during the state change of energy absorptivematerial 62, energy absorptive material 62 is serving to at leastpartially limit a further rise in the temperature of components ofexternal antenna 52. As the state change temperature of energyabsorptive material has been preferably selected to be near or justbelow the temperature limit of the skin of patient 18, energy absorptivematerial 62 will tend to limit the temperature components of externalantenna 52 from reaching the temperature limit and, hence, will alsotend to limit the temperature of the skin of patient 18 from reachingthe maximum desired temperature limit.

In a preferred embodiment, energy absorptive material 62 is constructedfrom wax and, in particular, a wax which has change of state temperatureof approximately the maximum temperature at which external antenna 52 isdesired to reach, such as thirty-eight (38) or thirty-nine (39) degreesCentigrade. Thus, it is preferred that the wax material of which energyabsorptive material is constructed melt at that temperature.

Inductive coupling between primary coil 54 of external antenna 52 andsecondary coil of implantable medical device 16 is accomplished at adrive, or carrier, frequency, f_(carrier), in the range of from eight(8) to twelve (12) kiloHertz. In a preferred embodiment, the carrierfrequency f_(carrier), of external antenna 54 is approximately nine (9)kiloHertz unloaded.

However, the inductive coupling between primary coil 54 of externalantenna 52 and secondary coil 34 of implantable medical device isdependent upon the mutual inductance between the devices. The mutualinductance depends upon a number of variables. Primary coil 54 ispreferably made from a coil of wire that has an inductance L and aseries or parallel tuned capacitance C. The values of both inductance Land capacitance C are fixed. For instance, if the desired drivefrequency, f_(carrier), of the energy transfer system was to be 1megaHertz and external antenna 52 had an independence of one microHenry,capacitance would be added so that the resonant frequency of the energytransfer system would equal that of the drive frequency, f_(carrier).The total capacitance added can be found using the equation f_(resonate)equals one divided by two times pi (π) times the square root of L timesC where L is the inductance of the energy transfer system. In thisexample, the value of capacitance C required to tune external antenna 52to resonate at the carrier frequency of 1 megaHertz is calculated asapproximately 25 nanofarads.

However, when the electrical properties of external antenna 52 change,either by the reflected environment or due to a physical distortion orchange in the composition of the external antenna 52, the inductance, L,may be altered. The inductance, L, can be altered because it is made upof two separate parts. The first part is the self-inductance, L_(self),of external antenna 52 at f_(carrier). The second part is the mutualinductance, L_(mutual), which is a measure of the change in currentdriving external antenna 52 and the magnetic effect, or “loading”, whichthe environment has on external antenna 52. When the electricalcharacteristics of the environment of external antenna 52 change,L_(self) remains constant while L_(mutual) varies. The effect of achange in the overall inductance, whether that change is from L_(self)or from L_(mutual), is a change in the resonant frequency, f_(resonate).Since C was chosen in order to have the resonant frequency,f_(resonate), match the drive frequency, f_(carrier), in order toincrease the efficiency of energy transfer from primary coil 54 ofexternal antenna 52 to secondary coil 34, a change in either or canresult in the resonant frequency, f_(resonate), being mismatched withthe drive frequency, f_(carrier). The result can be a less than optimumefficiency of energy transfer to implantable medical device 16.

As the drive frequency, f_(carrier), varies with respect to the resonantfrequency, f_(resonate), apparent impedance of the energy transfersystem, as seen by primary coil 54, will vary. The apparent impedancewill be at a minimum when the drive frequency, f_(carrier), exactlymatches the resonant frequency, f_(resonate). Any mismatch of the drivefrequency, f_(carrier), from the resonant frequency, will cause theimpedance to increase. Maximum efficiency occurs when the drivefrequency, f_(carrier), matches the resonant frequency, f_(resonate).

As the impedance of the energy transfer system varies, so does thecurrent driving primary coil 54. As the impedance of the energy transfersystem increases, the current driving primary coil 54 will decreasessince the voltage being applied to primary coil 54 remains relativelyconstant. Similarly, the current driving primary coil 54 will increaseas the impedance of the energy transfer system decreases. It can be seenthen that point of maximum current driving primary coil 54 will be at amaximum when the impedance of the energy transfer system is at aminimum, when the resonant frequency, f_(resonate), matches the drivefrequency, f_(carrier), and when maximum efficiency occurs.

The impedance of the energy transfer system can be monitored since thecurrent driving primary coil 54 varies as a function of drive frequency,f_(carrier). The drive frequency can be varied and the current drivingprimary coil can be measured to determine the point at which theimpedance of the energy transfer system is at a minimum, the resonantfrequency, f_(resonate), matches the drive frequency, f_(carrier), andwhen maximum efficiency occurs.

In a preferred embodiment, instead of holding the drive frequency,f_(carrier), constant for a nominal resonant frequency, f_(resonate),the drive frequency, f_(carrier), is varied until the current drivingprimary coil 54 is at a maximum. This is not only the point at which theimpedance of the energy transfer system is at a minimum but also thepoint at which maximum efficiency occurs.

Maximum efficiency is not as important in systems, such as telemetrysystems, which are utilized in a static environment or for relativelyshort periods of time. In a static environment, the resonant frequency,f_(resonate), may be relatively invariable. Further, efficiency in notterribly important when energy or information transfer occurs over arelatively short period of time.

However, transcutaneous energy transfer systems can be utilized overextended periods of time, either to power the implanted medical device16 over an extended period of time or to charge a replenishable powersupply within implanted medical device 16. Depending upon capacity ofthe replenishable power supply and the efficiency of energy transfer,charging unit 50 can be utilized for hours and typically can be used aspatient 18 rests or over night as patient 18 sleeps. Further, over theextended period of time in which charging unit 50 is utilized, externalantenna 52 is affixed to the body of patient 18. As patient 18 attemptsto continue a normal routine, such as by making normal movement or bysleeping, during energy transfer, it is difficult to maintain externalantenna 52 in a completely fixed position relative to secondary coil 34.Movement of external antenna 52 with respect to secondary coil 34 canresult in a change in mutual inductance, L_(mutual), a change inimpedance and a change in the resonant frequency, f_(resonate). Further,any change in spatial positioning of the energy transfer system with anyexternal conductive object, any change in the characteristics ofexternal antenna 52, such as by fractures in magnetic core 58, forexample, a change in the charge level of rechargeable power source 24 ofimplantable medical device 16 or a change in the power level of chargingunit 50, all can result in a change of mutual inductance, L_(mutual).

In a preferred embodiment, drive frequency, f_(carrier), is varied, notonly initially during the commencement of energy transfer, e.g.,charging, but also during energy transfer by varying the drivefrequency, f_(carrier), in order to match the drive frequency, with theresonant frequency, f_(resonate) and, hence, maintaining a relativelyhigh efficiency of energy transfer. As an example, drive frequency,f_(carrier), can be constantly updated to seek resonant frequency,f_(resonate), or drive frequency, f_(carrier), can be periodicallyupdated, perhaps every few minutes or every hour as desired. Suchrelatively high efficiency in energy transfer will reduce the amount oftime charging unit 50 will need to be operated, for a given amount ofenergy transfer, e.g., a given amount of battery charge. A reducedenergy transfer, or charging, time can result in a decrease in theamount of heating of implanted medical device 16 and surrounding tissueof patient 18.

In a preferred embodiment, external charging device 48 incorporatestemperature sensor 87 in external antenna 52 and control circuitry incharging unit 50 which can ensure that external antenna 52 does notexceed acceptable temperatures, generally a maximum of thirty-eightdegrees Centigrade (38° C.). Temperature sensor 87 in external antenna52 can be used to determine the temperature of external antenna 52.Temperature sensor 87 can be positioned in close proximity to thermallyconductive material 62 in order to obtain reasonably accurateinformation on the temperature of the external surface of externalantenna 52 contacting patient 18. Preferably, temperature sensor 87 isaffixed to thermally conductive material 62 with a thermally conductiveadhesive. Thermally conductive material 62 smoothes out any temperaturesdifferences which otherwise might occur on the surface of externalantenna 52 contacting patient 18. Positioning temperature sensor 87 inthe proximity or touching thermally conductive material 62 enables anaccurate measurement of the contact temperature.

Control circuitry using the output from temperature sensor 87 can thenlimit the energy transfer process in order to limit the temperaturewhich external antenna 52 imparts to patient 18. As temperature sensor87 approaches or reaches preset limits, control circuitry can takeappropriate action such as limiting the amount of energy transferred,e.g., by limiting the current driving primary coil 54, or limiting thetime during which energy is transferred, e.g., by curtailing energytransfer or by switching energy transfer on and off to provide an energytransfer duty cycle of less than one hundred percent.

When the temperature sensed by the temperature sensor is well belowpreset temperature limits, it may be acceptable to report thetemperature with relatively less precision. As an example, if thetemperature sensed by temperature sensor 87 is more than two degreesCentigrade (2° C.) away from a preset limit of thirty-eight degreesCentigrade (38° C.), it may be acceptable to know the temperature withan accuracy of three degrees Centigrade (3° C.).

However, when the temperature of external antenna 52 approaches towithin two degrees Centigrade (2° C.), it may be desirable to know thetemperature with a much greater accuracy, for example, an accuracy ofwithin one tenth of one degree Centigrade (0.1° C.).

It is generally difficult, however, to produce a temperature which has ahigh degree of accuracy over a very broad temperature range. While atemperature sensor can easily be produced to provide a resolution withinone-tenth of one degree Centigrade (0.1° C.) over a relatively narrowrange temperatures, it can be difficult to produce a temperature sensorproviding such a resolution over a broad range of temperatures.

In a preferred embodiment, a dual range temperature sensor is utilized.This temperature sensor has a first, broad, less accurate range ofmeasurement from thirty-one degrees Centigrade (31° C.) to forty degreesCentigrade (40° C.) having an accuracy within three degrees Centigrade(3° C.). Further, this temperature sensor has a second, narrow, moreaccurate range of measurement over four degrees Centigrade (4° C.), fromthirty-six degrees Centigrade (36° C.) to forty degrees Centigrade (40°C.), having an accuracy within one-tenth of one degree Centigrade (0.1°C.).

FIG. 16 illustrates a preferred embodiment of a dual range temperaturesensor utilizing temperature sensor 87. Temperature sensor 87, locatedin external antenna 52, is coupled to amplifier 170 which has beenpre-calibrated to operate only in the range of from thirty-six degreesCentigrade (36° C.) to forty degrees Centigrade (40° C.). Components ofamplifier 170 have an accuracy reflecting a temperature within one-tenthof one degree Centigrade (0.1° C.). The analog output of amplifier 170is sent to analog-to-digital converter 172 producing a digital output173 having an accuracy of one-tenth of one degree Centigrade (0.1° C.).The analog output of amplifier 170 is also sent to comparator 174 whichcompares the analog output against a known reference voltage 176 whichis set to at a predetermined level to produce a positive output 178 whentemperature sensor 87 reflects a temperature of thirty-eight degreesCentigrade (38° C.), the maximum temperature permitted for externalantenna 52. Control logic in charging unit 50 can then take appropriateaction to limit further temperature increases such as by ceasing orlimiting further energy transfer and/or charging. Temperature sensor 87is also coupled to amplifier 182. Components of amplifier 182 have anaccuracy reflecting a temperature within three degrees Centigrade (3°C.), much less accuracy than amplifier 170, but amplifier 182 canoperate over the much larger temperature range of thirty-one degreesCentigrade (31° C.) to forty-five degrees Centigrade (45° C.). Theoutput of amplifier 182 is sent to analog-to-digital converter 184producing a digital output 186 having an accuracy of three degreesCentigrade (3° C.).

Some or all of the various features of implantable medical device 16 andcharging unit 50 described enable a system for transcutaneous energytransfer having a relatively high efficiency of energy transfer,especially in situations involving some latitude of maladjustment ofexternal antenna 52 with secondary coil 34. High efficiency of energytransfer can enable a rechargeable power source 24 of implantablemedical device 16 to be charged, or recharged, within a shorter periodof time than would otherwise be possible. Alternatively or in addition,high efficiency of energy transfer can enable transcutaneous energytransfer to occur at higher rate than would otherwise be possible sincemore of the energy generated by charging unit 50 is actually convertedto charging rechargeable power source 24 instead of generating heat inimplanted medical device 16 and/or surrounding tissue of patient 18.Alternatively or in addition, high efficiency of energy transfer canresult in lower temperatures being imparted to implanted medical device16 and/or surrounding tissue of patient 18. Alternatively or inaddition, high efficiency of energy transfer can enable a greater degreeof maladjustment of external antenna 52 with secondary coil 34effectively resulting in patient 18 being able to be more ambulatory.

Thus, embodiments of the external power source for an implantablemedical device having an adjustable magnetic core and system and methodrelated thereto are disclosed. One skilled in the art will appreciatethat the present invention can be practiced with embodiments other thanthose disclosed. The disclosed embodiments are presented for purposes ofillustration and not limitation, and the present invention is limitedonly by the claims that follow.

What is claimed is:
 1. A system, comprising: an implantable medicaldevice adapted to be implanted within a patient, comprising: arechargeable power source; and a secondary coil coupled to therechargeable power source; and an external device, comprising: a housinghaving a surface adapted to assume a position in proximity to a surfaceof the patient during recharging of the rechargeable power source; aprimary coil adapted to be transcutaneously coupled to the secondarycoil during the recharging; and a temperature sensor in said externaldevice and adapted to sense a temperature of the surface of the housingduring the recharging; and control circuitry to limit energy transferbased on the temperature of the surface of the housing to limit atemperature of the external device to no more than a predeterminedtemperature.
 2. The system of claim 1, wherein the control circuitry isadapted to limit energy transfer to the implantable medical deviceduring the recharging based on the temperature of the surface of thehousing.
 3. The system of claim 2, wherein the control circuitry isadapted to limit energy transfer to the implantable medical deviceduring the recharging by limiting time during which energy istransferred.
 4. The system of claim 3, wherein the control circuitry isadapted to switch energy transfer on and off to provide an energytransfer duty cycle.
 5. The system of claim 1, wherein the controlcircuitry comprises a processor.
 6. The system of claim 1, wherein theexternal device further comprises a circuit to vary a frequency of arecharging signal to substantially optimize efficiency of energytransfer between the primary coil and the secondary coil.
 7. The systemof claim 1, wherein the external device further comprises thermallyconductive material in proximity to the temperature sensor.
 8. Thesystem of claim 1, wherein the external device further comprises atelemetry coil adapted to exchange communication signals with theimplantable medical device.
 9. The system of claim 1, wherein thesurface is adapted to contact skin of the patient during recharging ofthe rechargeable power source, and wherein said output from thetemperature sensor is indicative of a temperature of the skin of thepatient.
 10. A system, comprising: an implantable medical deviceimplantable within a patient and comprising a secondary coil; and anexternal device to transcutaneously transfer energy to the secondarycoil, the external device comprising: a housing having a side adapted tobe positioned in proximity to the secondary coil when the externaldevice is transferring energy to the secondary coil; a temperaturesensor in said housing and coupled to the side to determine atemperature indicative of heat to which the patient is being exposedduring the transfer of energy; and a control circuit adapted to limitthe transfer of energy to the secondary coil based on the temperature;wherein the control circuit is adapted to limit the transfer of energyso that the temperature does not exceed a maximum temperature.
 11. Thesystem of claim 10, wherein the control circuit is adapted to limit timeduring which the energy is transferred based on the temperature.
 12. Thesystem of claim 10, wherein the external device further comprises aprimary coil, and wherein the control circuit is adapted to controlamplitude of a signal within the primary coil based on the temperature.13. The system of claim 10, wherein the temperature sensor isthermally-coupled to a surface of the side of the housing, the surfacebeing in contact with the patient.
 14. The system of claim 10, whereinthe control circuit further comprises a processor executing programmedinstructions, and wherein the predetermined maximum temperature isselectable by the processor.
 15. The system of claim 10, wherein theexternal device further comprises a circuit to vary a frequency of asignal used to transfer energy to the secondary coil to substantiallyoptimize efficiency of the transfer of energy.
 16. The system of claim10, wherein at least a portion of the side of the housing carries athermally-conductive material.
 17. The system of claim 10, wherein theimplantable medical device further comprises circuitry to delivertherapy to the patient and magnetic material positioned between thesecondary coil and at least a portion of the circuitry.
 18. The systemof claim 10 wherein said side of said external device is configured tobe placed against skin of a patient, and wherein the temperatureindicative of heat to which the patient is being exposed during thetransfer of energy is indicative of a temperature of said skin of saidpatient.